Knowledge of the chemical transfer and mineralogical transformations that occur when sea water reacts with basalt at elevated temperatures and pressures can be used along with geological and geophysical data to deduce the typical structure and evolutionary sequence for hydrothermal systems within the oceanic crust along the axis of a mid-ocean ridge. Studies of metabasalt and metadiabase dredged from fault scarps along the axial valley of the Mid-Atlantic Ridge reveal consistent relationships among the bulk chemistry of the altered rocks and their secondary mineralogy, mineral abundances, and mineral compositions, especially for chlorite. These relationships can be interpreted in terms of the distribution of alteration with respect to time, temperature, and water/rock ratio in young crust. Assemblages of chl-ab-ep-act, chl-ab-ep-act-qtz, chl-ab-qtz, and chl-qtz are produced at successively higher effective sea-water to rock ratios within the temperature range of the greenschist facies (−250 to 450 °C). Toward higher ratios, chlorites tend to become more Mg-rich. Pillow basalts of layer 2 are commonly altered under these conditions by sea water on the descending, rather than the ascending, limb of a convection system; this type of alteration, which reflects moderately high water/rock ratios, may characteristically occur above a still partially molten magma chamber. As the magma chamber solidifies and is then penetrated, more pervasive alteration of the deeper part of layer 2 and of layer 3 occurs at lower water/rock ratios, producing solutions such as those recovered from springs on the Galapagos Rift and the East Pacific Rise at 21°N. Localized upwelling limbs of the convection system can produce veins filled with quartz, sulfides, and Fe-rich chlorite.